Radiation standard source



'1 L- fcounter tube in order to obtain accurate results.

' often, thestandard radiation sources are either particles of r-adioisotopes or their salts 'or are liquid solutions. Be-

cause of their nature, these standard sources are usually of small size and must be placed into adapters designed to correctly position'the standard source with relation to the instrument. These standard sources are often difiicult l 3,133,025 V 1 RADIATION STANDARD soUucE Donald C. Walker, Lansing, Ill., and Bernard A. Girman,

EastG'ary, ma, assignors to Standard Oil Company,

Chicago, 111., a corporation of Indiana No Drawing.

FiledMay 4, 19m, $61. Nb. 26,673 1 Claim.

Thisinvention relates to radoactive materials. More particularly it relates to radioactive solids suitable for use as standard sources of radiation.

Standard radioactive sources find many uses in industry.

' One particular use is in the calibration of ionization chamber type radiation measuringinstruments, such as Geiger- Miiller tubes, which require frequent calibration because of changes occurring in the characteristics of the various instrument components. In calibrating these instruments the counting rate of a standard radiation source of a known amount and known .specific activity is used as a standard.

QTo accurately calibrate an instrument the standard source must be placed in an exact position with relation .to the to prepare and frequently are quite expensive.

uted. essentially uniformly throughout the body. A particular'object of thisinvention is a standard radiation source suitable for use in calibrating a radiation'counting tube and adapted to be fabricated to accurately fit over We have found that radioactive solid materials may be prepared by distributing essentially uniformly a radio- 2 7 active material throughout the body of a solid resinous material. "tially uniformly throughout the body of a liquid resinous The radioactive material is distributed essenrnaterial which is then converted to a solid material by a polymerization-type reaction to provide a solid resinous L ,substance' having an essentially uniform counting rate.

7 The resinous materials which may be used in the practice ofthe invention are those plastics and resins which 7 may be liquified at a temperature lower than the decomposition temperature of the radioactive material used, or those resins which are normally liquid and are capable "of being converted to a'solid. material by a polymerizae 'tion-type reaction. There are a number of useful resinous substances, normally solid, but liquifiable by heating Among these ma terials. are polyvinyl chloride, polyvinyl acetate, poly- .methyl methacrylate, polystrene and the polyolefins such 'as polyethylene, polypropylene, and polyethylenepolypropylene copolymers.

Other types of resins may also be used, such as those i -;Wherein-a liquid monomer is polymerized to form a solid 1 material. Examples are the styrene-type resins, such as vinyl benzene, di-vinyl benzene and a-methyl styrene which are polymerized by heating in the presence of a Most ";.- U 'td li t Paint Patented May 12, 1964 peroxy catalyst. The normally liquid epoxy intermediate resins may also be used. These resins are cured to form a solid material, without heatingby the addition, of an amine or acid activator. Epoxy resins suitable for this purpose are described the publication Epoxy Resins by Irving Slciest, Reinhold Publishing Corporation (1958).

In view of the large number of resinous: substances available, the above resins" represent only typical categories of suitable resinous materials, and'othe'rs of this type may be used in the practice of the invention. The I 7 physical and mechanical properties of a particular resin will determine its usefiulnessfor a particular application. In general, however, it has been found that resins which possess a high degree of hardness and rigidity are the most suitable.

The radioactive material may be a source of alphabeta or gamma radiation, however, from the standpoint of safety and ease of handling, a weak beta emitter is generally satisfactory. The radioactive material should be capable of being uniformly distributed throughout: the

body of the resinoussubstance. For instance, an insolu- 'ble'radiotracer may be. used in finely divided form and distributed throughout the resin by milling. .However, it is preferred to use a radiotracer which is soluble in a liquid resin. The characteristics of the radioactive materialmay vary according to the particular application, but

most often,'it is preferred to employ a radioisotope having a long half-life. Carbon-14 emits a weak beta radiation having an energy of about 0.155 mev. and has a halflife of about 5600 years. Synthesized organic compounds labeled with the carbon-l4 radioisotope are available and provide a number of radioactive materials which may be employed. "Other radioisotopesor radioactive materials which may be uniformly distributed throughout theresin may be employed, according to the particular application.

The invention is particularly well adapted to the in-. stance where the radioactive material is soluble in the resin used. In this instance, the choice of the particular I radioactive material will depend upon the resin which is to be used. The epoxy resins are preferred for many 1. the'window of such a tube. Another object of this inven- 1 tion is 'amethod of obtaining a true cross-section counting rate of a solution containing a polar radioisotope.

applications because oftheir desirable properties. It has been found that the carbon-14 labeled compounds of the stearyl group are well adapted touse in conjunction with theseresins. Compounds such as radiostearyl alcohol, radio-a-hydroxystearic'acid and radiostearic acid have been found to be particularly suited, with radiostearic acid being preferred. a

The concentration of the radioactive material dissolved in the resin may vary considerably, depending upon the specific activity desiredin the standard source. The

' specific activity of the radioactive material of course, will influence the amount used. Where a compound such as described above is used, it has been foundthat the number of counts per minute from the standard source varies in direct proportion to the concentration of the radioisotope dissolved in the liquid resin. If the specific activity of the radioisotope is unknown, it may be determined by preparing a dilute molal solution with a resin,

forming a disc by polymerizing the resin and then comparing the disc to another disc containing an equal molal concentration of an isotope of a known activity. Thus, many radiotracers may be used as described above and the concentration of the radiotracer employed will be dependent upon the nature ofthe tracer and the desired specific activity.

As an example of the ease with which the standard radiation source of the invention may be prepared, standard sources were made up 'using radiostearic acid-l-Cp; and an amine-activated epoxy resin. Four discs were prepared by this method using radiostearic acid of about 3 one millicurie per millimole specific activity. Varying amounts of the radiostearic acid were dissolved in the liquid epoxy resin, as shown in Table I, by gently heating and stirring. The resin was then polymerized at room temperature by adding an amine activator, and the counting rate determined.

Table 1 Percent radiostearic acid Counts per minute 0.04 27,300 0.02 13,700 0.01 6,800 0.005 3,400

Thus, it been that the counting rates of the solid resin containing the dissolved radiostearic acid are in direct proportion to the amount of radioisotope present in the resin.

The discs were prepared by casting the resin in an aluminum ring to form a disc approximately /1" thick. After polymerization the disc was then chucked in a lathe and one side was faced otf to give a smooth fiat surface.

Subsequently, the machined'surface of the disc was polished using Carborundum paper on glass. The discs thus prepared were found to be very suitable for calibrating a Geiger-Miiller tube, and were tound to possess the uniform activity across the cross-section of the disc. While the disc-form of standard source is generally preferred, any shape planar body may be used. For example, a disc may be machined to a configuration adapted liquid resin by the method described herein and the resin polymerized to form a solid the isotope molecules are then immobilized. By machining away the surface, or the interface, any surface concentration is cut away and a true cross-section counting rate may be readily obtained by the use of a counting tube.

Having described our invention, what we claim is:

A radioactive solid solution having an essentially uniform specific activity which solid solution consists essentially of an amine-activated epoxy resin having from about 0.005 to about 0.05% radiostearic acid of about one millicurie per millimole specific activity dissolved in said resin.

References Cited in the file of this patent UNITED STATES PATENTS Uses of Atomic Energy, vol. 15, pp. 307-309. (1955). Schweitzer et al.: Nucleonics, vol. 7, No. 3, pp. 65-66, September 1950.

Aglintsev et al.: 2nd US. Conference on Peaceful Uses of Atomic Energy, vol. 21, pp. 83-89, September 1-13, 1958. 

